Synthesis of oligomethylene-strapped chlorophyll derivatives and optical properties of their stereoisomers in a solution

Methyl pheophorbide-a/a′ derivatives covalently linked with oligomethylene chains at the 3-CH2OCO– and 132-COO– moieties in a molecule were prepared by modifying chlorophyll-a through intramolecular ring-closing metathesis of vinyl groups. At least, a C10-length between the 33- and 134-positions was necessary for the cyclization and connection of a C12-strap was the most suitable to achieve the highest closure yield. The oligomethylene chain in 132R-epimers derived from methyl pheophorbide-a covered the α-face of the chlorin π-plane and the strap in the corresponding 132S-epimers protected the β-face. Synthetic 132R-epimer with a dodecamethylene chain gave a flat chlorin π-plane, while the decamethylene chain in the 132R-epimer distorted the π-system due to its shorter linkage. The distortion by strapping in the 132R-epimer induced a slight blue-shift of Qy peak in dichloromethane. CD spectra of the 132R-epimers were similarly dependent on the chain length, i.e., the distortion of π-plane. Visible absorption and CD spectra of all the strapped 132S-epimers were almost identical and only slightly different from those of the unstrapped. The strapping in the 132S-epimers shifted the Qy peak bathochromically.